Memory is one type of integrated circuitry, and is used in computer systems for storing data. Integrated memory is usually fabricated in one or more arrays of individual memory cells. The memory cells might be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time, and in some instances can store data in the absence of power. Non-volatile memory is conventionally specified to be memory having a retention time of at least about 10 years. Volatile memory dissipates and is therefore refreshed/rewritten to maintain data storage. Volatile memory may have a retention time of milliseconds, or less.
The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
Nonvolatile memory may be used in applications in which it is desired to retain data in the absence of power. Nonvolatile memory may also be used in applications in which power is a limited resource (such as in battery-operated devices) as an alternative to volatile memory because nonvolatile memory may have the advantage that it can conserve power relative to volatile memory. However, read/write characteristics of nonvolatile memory may be relatively slow in comparison to volatile memory, and thus volatile memory is still often used, even in devices having limited reserves of power. It would be desirable to develop improved nonvolatile memory and/or improved semi-volatile memory. It would be further desirable to develop memory cells that are nonvolatile or semi-volatile, while having suitable read/write characteristics to replace conventional volatile memory in some applications.
Integrated circuit fabrication continues to strive to produce smaller and denser integrated circuits. It can be desired to develop small-footprint memory cells in order to conserve the valuable real estate of an integrated circuit chip. For instance, it can be desired to develop memory cells that have a footprint of less than or equal to 4F2, where “F” is the minimum dimension of masking features utilized to form the memory cells.
One type of memory cell is a thyristor-based random access memory (T-RAM) cell. A thyristor is a bi-stable device that includes two electrode regions (an anode region and a cathode region) and two base regions between the electrode regions. The four regions are alternating p-type and n-type regions. For instance, an example configuration may have a p-type anode region, an n-type base, a p-type base, and an n-type cathode region arranged in a p-n-p-n configuration. A thyristor includes two main terminals, one at the anode region and one at the cathode region, and includes a control terminal. The control terminal is often referred to as a “gate,” and may be electrically coupled with one of the base regions (conventionally, the gate is coupled to the base region nearest the cathode).
A thyristor in a memory device may be turned on by biasing the gate so that a p-n-p-n channel conducts a current. Once the device is turned on, often referred to as “latched,” the thyristor does not require the gate to be biased to maintain the current conducted between the cathode and the anode. Instead, it will continue to conduct until a minimum holding current is no longer maintained between the anode and cathode, or until the voltage between the anode and the cathode is reversed. Accordingly, the thyristor may function as a switch or diode capable of being switched between an “on” state and an “off” state.
T-RAM cells may have faster switching speeds and lower operating voltages than conventional SRAM cells. However, T-RAM cells may also have lower than desired retention times, and may have a large footprint.
It would be desired to develop new memory cells which can be non-volatile or semi-volatile, and which have may have a footprint approaching 4F2.